This invention relates to a method and apparatus for color-image processing and, more particularly, to a color-image processing method and apparatus in which entered image signals separated into the three primary colors are converted into color signals when a visible image is outputted in the form of a hard copy.
In a color-image processing apparatus of this type known in the prior art, separated color signals entered as R, G and B usually are converted into color signals Y (yellow), M (magenta), C (cyan) and K (black), which constitute a subtractive mixture of color stimuli, by an arrangement of the kind shown in FIG. 1, and the resulting color signals are outputted in order to obtain a hard copy of the separated color signals.
As illustrated in FIG. 1, the three primary-color signals R, G, B entered from signal lines 101 are converted into complementary colors C, M, Y by a logarithmic converting circuit 102, and the signal K, which is the minimum value of C, M, Y [i.e., K=min(C,M,Y)], is produced by a minimum-value extracting circuit 103. The complementary C, M, Y signals resulting from the conversion are transformed into signals C', M', Y' by a matrix transformation circuit 104, generally referred to as a masking circuit, in order to correct unnecessary absorption components of the pigments used in a hard-copy output apparatus. Here a transformation equation indicated by Equation (1) below holds, where A.sub.ij represents a predetermined coefficient. ##EQU1##
The C', M', Y' signals obtained in accordance with Equation (1) are output signals for obtaining a hard copy of the input signal. Since black is reproduced by mixing the C', M', Y' signals together in equal amounts, black usually is substituted for part of C', M', Y' by processing referred to as UCR (undercolor removal).
A UCR circuit 105 produces four primary-color output signals C", M", Y", K" from C', M', Y' and K' in accordance with Equation (2) below, in which B.sub.i represents a predetermined coefficient. These final signals are outputted on signal lines 106. EQU C"=C'-B.sub.1 .times.K EQU M"=M"-B.sub.2 .times.K EQU Y"=Y'-B.sub.3 .times.K EQU K"=B.sub.4 .times.K . . . (2)
The C", M", Y", K" signals generated in accordance with Equation (2) are sent to a printer output unit (not shown), which executes the above-described processing in pixel units to form a visible color image.
However, in the example of the prior art set forth above, the coefficients B.sub.i used in Equation (2) generally are values not uniquely defined. That is, it is possible for C', M', Y' to be replaced by K in a certain percentage. If B.sub.i =1 holds, the components corresponding to the minimum values among C', M', Y' will all be replaced by K. This generally is referred to as 100% UCR. If B.sub.i =0 holds, UCR is not performed. In many cases Bi takes on a value between 0 and 1.
As obvious from the foregoing description, at B.sub.i =1 the printer output unit is capable of effecting substitution by the K signal since 100% UCR is applied. However, since substitution by the K signal is carried out even at flesh-color portions and highlight-color portions of pastel colors, there is a decline in overall hue and color reproducibility.